The field of the invention is the production of alkyl esters of saturated aliphatic carboxylic acids and the present invention is particularly concerned with reacting olefins with carbon monoxide and alkanol in the presence of a catalyst consisting of a cobalt compound and a promoter selected from pyridine, non-ortho-substituted alkylpyridine or mixtures thereof at elevated pressures and elevated temperatures.
The state of the art of such alkoxycarbonylation reactions may be ascertained by reference to U.S. Pat. Nos. 3,507,891; 3,906,016; 3,976,670 and 4,041,057 and the article "Hydrocarboxymethylation--an Attractive Route from Olefins to Fatty Acid Esters?" by Peter Hofmann et al as published in I & EC, Product Research & Development, Vol. 19, September 1980, pp. 330-334, the disclosures of which are incorporated herein.
For comparison purposes alkoxycarbonylation is compared in the present invention with hydroformylation as disclosed in U.S. Pat. No. 4,320,237 the disclosure of which is incorporated herein.
It is known that by reacting olefins with carbon monoxide and a compound having a replaceable hydrogen atom such as an alkanol in the presence of a catalyst containing a metal of Group VIII of the Periodic Table of elements and possibly a promoter, fatty acid esters can be produced as disclosed in J. Falbe, Synthesen mit Kohlenmonoxid, Springer, publishers, Berlin, Heidelberg, New York (1967).
A preferred variation of this reaction known as alkoxycarbonylation, is the conversion in the presence of cobalt catalysts. An especially preferred implementation with cobalt catalysts uses additionally pyridine or a non-ortho-substituted alkylpyridine as a promoter as disclosed in U.S. Pat. No. 3,507,891 and U.S. patent application Ser. No. 125,482 filed Feb. 28, 1980.
The alkoxycarbonylation reaction catalyzed by cobalt-pyridine or cobalt pyridine derivatives is similar in its chemistry, its starting materials required for the conversion, the conditions of reaction and the exothermal heat of reaction released, to other carbonylation reactions, in particular the hydroformylation of olefins.
Thus the catalytic cycles of alkoxycarbonylation and hydroformylation differ only in the last stage when the end product is formed. During the aldehyde formation by hydrogenating the C--Co bond of the acyl complex, the ester formation takes place by the alcoholysis of this bond as follows: ##STR1## where x+y=3 or 4; L=CO or pyridine or a pyridine derivative.
The mixtures of starting materials in alkoxycarbonylation and hydroformylation as a rule consist of those components which will be liquid under the pressures used in the reaction and also those which are still gaseous under these conditions.
Hardly any difference exists regarding optimum pressure and temperature ranges for an alkoxycarbonylation catalyzed by cobalt and pyridine or cobalt and pyridine derivatives and a hydroformylation catalyzed by the same metal. In both cases the conventionally observed temperatures and pressures are within the ranges of 130.degree. to 230.degree. C. and 100 to 300 bars.
Both reactions are exothermal and the particular heats of reaction as a rule are in a range from 28 to 35 kcal/mole.
In view of this wide analogy between these two reactions, it is not surprising that identical reactor designs have been proposed for the continuous, large industrial-scale alkoxycarbonylation and hydroformylation as disclosed in German Pat. No. 926 846, page 2, lines 95 to 117. As a rule these reactors are cylindrical reaction vessels wherein intensive mixing of the reactor contents is achieved by the incorporation of concentric guide pipes and by the introduction of the starting materials, for instance, by high nozzle speed injections at one or several sites along the reactor. A series of steps is known whereby the typical agitation vessel features of the carbonylation reactors are achieved as disclosed by (H. Dubil, J. Gaube, Chem. Ing. Techn. 45 (8), 529-533 [1973]) and in the method of West German Published Application 11 35 879 a liquid is made to circulate by heat convection around a circulation pipe which is freestanding inside the reactor. As regards the method of West German Published Application 10 85 144, the reactor contents are made to circulate by high gas loads using the principle of the air lift pump. In the method of British Pat. No. 1,079,209 the circulation of the liquid is achieved by using an impulsive introduction of the high nozzle-speed reagents. The agitation vessel behavior can also be adjusted by the method of West German Pat. No. 10 03 708, that is by repumping the liquid reactor contents.
Even though reaction losses always had to be incurred as a consequence of the characteristic dwell-time behavior related to the agitated vessel when the above reactor design is maintained in the absence of any special supplemental steps, the reactor with back-mixing has gained widespread use in carrying out carbonylation reactions in industry. A similar process for the production of acetic acid by carbonylating methanol is disclosed in J. Falbe, Synthesen mit Kohlenmonoxid, Springer publishers, Berlin-Heidelberg-New York, 1967, p 118. The heat transfer conditions relating to this reactor having higher flow rates at the cooling surfaces and hence better control of the heat of reaction to be evacuated together with the back-mixing function permit the highest possible temperature constancy throughout the entire reaction space. Specific reaction conditions regarding the temperature of reaction, are assumed to be required for the optimum selectivity of the carbonylation reaction. It is only in the case where several isomeric products are formed that the optimum isomer ratio appears to be adjustable. Lastly a uniform temperature is expected to prevent any catalyst deactivation in zones of local overheating and at the same time also prevents the reaction from "going to sleep" at excessively low temperature sites. The above cited advantages are obtained at the cost of losses in reaction efficiency and of reactors which are more expensive to build.
U.S. Pat. No. 3,976,670 proposes a reactor concept for the continuous implementation of the alkoxycarbonylation reaction whereby it is possible to solve the problem of the heat removal also without back-mixing. In this method the reaction is carried out in a reactor with several intermediary feeders and consists of several segments which increase in diameter toward the end of the reaction tube as shown in FIG. 1 of the patent. The problem of heat evacuation is solved in that the alkanol not only is fed-in together with the other reagents at the beginning of the reactor, but that it also is simultaneously and additionally metered-in along the reactor at several places. Due to the ever limited local feeding of alkanol a more uniform reaction along the entire reactor is expected, and hence also a more uniform heat evacuation or removal and a longer catalyst life.
On the other hand, depending on how the reaction is carried out, the following drawbacks are incurred in the design of U.S. Pat. No. 3,976,670:
(1) Higher capital investment costs are incurred for a high-pressure reactor consisting of several segments of different diameters.
(2) The cost is higher to control the metering a plurality of flows of substances.
(3) In the range of shorter dwell-times which are particularly attractive for industrial applications because of the capacities involved, the reaction rates which are possible in the method of U.S. Pat. No. 3,976,670 are clearly less than the values which are possible using discontinuous batches in agitated autoclaves with simultaneous presence of all the reactants as illustrated in U.S. Pat. No. 3,976,670, at Examples 2 and 3, where single and double stoichiometric quantities of alcohol are added. The comparatively higher reaction rates that might result in the method of the U.S. Pat. No. 3,976,670 at longer dwell times are not economically significant because such a procedure clearly restricts the capacity as illustrated in German Published Application No. 19 63 804, (corresponding to South African Pat. No. 69/8771), page 14, 2nd paragraph.
(4) Lastly the application of the method of U.S. Pat. No. 3,976,670 is unquestionably bound to a clear loss in selectivity. This is so because a restriction on the local alkanol feed leads to an increase of the aldehydes and acetals formed as side products because of the required presence of only low quantities of hydrogen to achieve adequate rates of reaction. This is pointed out by Peter Hofmann et al ibid.; German Published Application No. 19 63 804, claim 1 and U.S. Pat. No. 3,507,891, column 3, lines 63-67.
Reactors permitting a continuous large-scale implementation of exothermal carbonylation processes therefore are designed extensively from the viewpoint of optimum heat evacuation or removal, as indicated above. As a result the above cited limitations and disadvantages will also inevitably follow and be accepted.